Stabilized polyphase bridge amplistat



June 22, 1965 F. W. KELLEY, JR

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June 22, 1965 F. w. KELLEY, JR 3,191,117

STABILIZED POLYPHASE BRIDGE AMPLISTAT Filed Oct. 27, 1960 3 Sheets-Sheet 2 PEA.

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STABILIZED POLYPHASE BRIDGE AMPLISTAT Filed 001;. 27, 1960 3 Sheets-Sheet 3 Fig.8.

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United States Patent 3,11,117 STABILFZED PEBLYPHASE BRIDGE AMPLISTAT Fred W. Kelley, Jr., Media, Pa., assignor to General Electric Company, a corporation of New York Filed Oct. 27, 1960, Ser. No. 65,428 Claims. (Cl. 323--89) This invention relates to magnetic amplifiers of the self saturating type having rectifiers connected in circuit with the gate windings of saturable reactor apparatus and generally known as amplistats. More particularly, the invention relates to poly-phase bridge connected amplistats provided with core structures that exhibit rectangular B-H loop characteristics and it has for an object the provision of means for improving the operation of magnetic amplifiers of this character. A more specific object of the invention is to improve the operating stability of polyphase bridge connected amplistats.

Heretofore such amplistats have exhibited different types of instability in their operating characteristics. Among the most troublesome types of instability are: (l) snap action instability; and (2) wave train instability. In snap action instability the control ampere turns vs. output characteristic has a region of discontinuity, with the result that within a certain range of control ampere turns the output of the amplifier has two values for each value of control ampere turns. This makes it possible for the amplistat to operate at either one of two output values depending upon the immediately preceding history of its operation. Furthermore, it prevents the amplistat from maintaining its output at aset value within a pre-determined range of values in the light load region of the characteristic.

Wave train instability is of an entirely different char- In the stable operation of a polyphase bridge connected amplistat, the output contains, for each cycle of the supply, a number of pulses equal to twice the number of phases of the amplistat and all such pulses are approximately uniform in amplitude and duration and have substantially the same firing angle. Wave train instability is characterized by different numbers of pulses in the outputfor different cycles of the supply and is further characterized by shifted firing angles. Thus, the average value of output current or voltage varies with time and exhibits some characteristics of a relatively low frequency oscillation. This modulates the load and precludes regulating the load voltage or current to a desired value.

Accordingly, a further object of this invention is the provision of improved polyphase bridge connected amplistat circuitry in which snap action instability and wave train instability are eliminated.

By way of a summary account of the invention there is provided a polyphase bridge connected amplistat comprising gate windings and D.-C. control signal windings associated with core material having a rectangular B-H loop characteristic. Each of the gate windings and D.C. control signal windings is associated with core material having a rectangular B-l-l loop characteristic. Each of the gate windings is connected with a different rectifier and with a load and a source of alternating voltage.

characteristic that is skewed with respect to the unmodified rectangular excitation characteristic of the amplistat alone.

The novel features which are believed to be character- "istic of this invention are set forth with particularity in "ice the appended claims. This invention, however, both as to its organization and method of operation together with further objects and advantages, may best be understood by reference to the following description taken in connection with the accompanying drawing in which:

FIG. 1 is a simple schematic diagram of an embodiment of the invention.

FIG. 2 is a plot of the idealized output vs. control current characteristic of an amplistat of the character described.

FIG. 3 is a plot of the output vs. control current characteristic of an amplistat of the character described operating under conditions of snap action instability and Wave train instability.

FIG. 4 is a chart of characteristic curves illustrating the character of the output of a polyphase bridge connected amplistat under conditions of wave train instability and under stable operating conditions.

FIG. 5 is a rectangular, B-H loop characteristic of a grain oriented nickel iron alloy commonly used as a core material in amplistats.

FIG. 6 is a graphical illustration of the exciting characteristics of an amplistat to which the invention is applicable and,

FIG. 7 is a chart of characteristic curves that is useful in facilitating an understanding of the construction of an embodiment of the invention.

Referring now to the drawing, and particularly to FIG. 1, the 3-phase bridge amplistat therein shown comprises six sections each containing a rectifier, a gate winding, and a control winding with both windings mounted on a common core member. Thus the amplistat comprises rectifiers 1, 2, 3, 4, 5 and 6, gate windings '7, 8, 9, 10, 11 and 12, and respectively associated control windings 7a, 8a, 9a, 10a, 11a and 12a. Each gate winding and its associated control winding are mounted on a separate core member. Thus, gate winding 7 and its associated control winding 7a are mounted on core member 13 and the remaining gate windings 8, 9, 1t), 11 and 12 and their associated control windings 8a, 9a, 10a, 11a and 12a are mounted on corresponding core members 14, 15, 15, 17 and 18 respectively. In series with each of the gate windings is connected one of the rectifiers. As shown, rectifier 1 is connected in series with gate winding 7 and the remaining rectifiers 2, 3, 4, 5 and 6 are similarly connected in series with gate windings 8, 9, 10, 11 and 12 respectively.

Dots associated with the gate windings and the control windings indicate the winding polarities. That is, if a voltage is applied to a gate winding such that the dot end is positive with respect to the opposite end, the voltage in duced in the associated control winding is positive at the dot end and negative at the opposite end.

The core members may be made of any one of a number of high flux density core materials having rectangular hysteresis loops, such for example as the grain oriented nickel iron alloys and cold rolled silicon steel tape. The dynamic B-H hysteresis loop characteristic of one of these materials, known to the trade and sold under the trade name Deltamax is illustrated in FIG. 5.

The cathodes of rectifiers 1, 2, and 3 are connected to a common conductor 19 which thus becomes the positive output terminal of the configuration. Similarly, the anodes of rectifiers 4, 5 and 6 are connected to a common conductor 20 which thus becomes the negative output The common terminals of the three sets or pairs of gate windings are connected to the 3-phase supply terminals L1, L2 and L3 respectively. The control windings 7a, 3a, 9a, 10a, 11a and 12a are connected in series relationship with each other to a source of D.-C. supply 23 and 24. Suitable means, such as an adjustable resistor 22, are provided for varying the current in the control windings.

The circuit configuration of the rectifiers and gate windings just described is generally known as a bridge amplistat.

Ideally, a bridge configuration amplistat with rectangular BIi hysteresis loop core material and a resistive load has an output v. control ampere turn characteristic generally similar to the relationship that is illustrated by curve 25 in FIG. 2 of which ordinates represent either output voltage or current and abscissae represent control winding current or ampere turns. With zero current in the control winding the amplistat is approximately fully turned on and the output is near maximum. As the control current is increased negatively the output diminishes to a minimum at point as, the abscissae of which is representative of a predetermined amount of negative current. As shown graphically in FIG. 2, a relatively wide range of control currents thus exercises control over the output from the amplistat to the lead.

A bridge amplistat with rectangular B-H loop characteristic core material has an exciting characteristic of which the curve 127 in FIG. 6 is typically representative. It has a rectangular shape that is generally similar to the rectangular B-H loop characteristic of its core material. In FIG. 6, ordinates represent the time integral of the voltage generated by the gate winding that is commonly referred to as the volt-seconds. Due to the abrupt saturating characteristic of the core material, the volt-seconds are limited to a maximum value that is equal to the prodnet of Ng, A, Br, and 10" in which:

Ng=number of turns of the gate winding A effective cross section of the core (in square centimeters) Br=the residual flux density in the core (in gausses).

Abscissae represent the gate winding current which in the case of an unsaturated core is limited to a value expressed by the fraction lHc .41rNg in which:

l=mean length of the ferromagnetic flux path (in centimeters) Hc=coercive force of core material in oersteds Ng=number of turns of gate winding.

In actual operation, an amplistat of the character described above will generally exhibit snap action instability and wave train instability as graphically illustrated by the output v. control ampere turns characteristic curve 28 in FIG. 3. In the light load region of the characteristic, such for example as in the region between points 28a and 28]) the amplistat typically exhibits wave train instability and in a neighboring region, such for example as between points 28c and 28d the amplistat may typically exhibit snap action instability. The regions of the output characteristic in which these instabilities are present may or may not overlap. In FIG. 3 they are illustrated as overlapping.

Wave train instability manifests itself in a variety of ways. One of the many manifestations of wave train instability is illustrated in FIG. 4 by the contrast between the output current characteristic illustrated in portion A of FIG. 4 by the series of pulses 29 representing an unstable wave train and the series of pulses 30 in portion B of FIG. 4 representing the stable wave train operating condition. The unstable operating condition pulses in portion A illustrate the output pulses occuring during two successive cycles of the supply source. Since the amplistat is a S-phase device and has a bridge rectifying configuration the wave train of its output pulses would be expected to contain a D.-C. component plus a sixth harmonic of the supply frequency plus various higher harmonics. Usually, such higher harmonics are attenuated in the system and are therefore not necessarily considered to be detrimental. The successive output current pulses 29 in portion A vary from instant to instant in amplitude, area and firing angle. Furthermore, strong harmonic components below the sixth harmonic are present. Such low frequency harmonics are not attenuated in the system and consequently they modulate the output causing random variations in the output that are highly undesirable. On the other hand, the stable operating condition pulses 3% shown graphically in portion B are uniformally equal in amplitude and area and their firing points occur at the same angle of retard in successive cycles. Consequently, the wave train does not produce any low frequency harmonics or oscillations in the output.

Concerning snap action instabl'ity as illustrated in FIG. 3, as the ampere turns of the control winding are increased in a negative sense or direction, the output current decreases in magnitude in accordance with curve 28 until it becomes discontinuous at point 28d. Any further slight increase in negative control ampere currents causes the gate current or voltage to decrease abruptly from the value represented by the ordinate I of point 28d to the value represented by the ordinate of point 31. Further increases in the negative control ampere turns produces a slight increase in the output voltage or' current along the curve 32 between points 31 and 28s. On the other hand, decreasing the negative ampere turns continuously results in a slight decrease in output current or voltage along curve 32 between points 282 and point 33. Any further slight decrease in negative control ampere currents at this point produces an abrupt increase in output current or voltage from the value represented by the ordinate of point 33 to the value represented by the ordinate of point 28c. This snap action instability of the amplistat prevents its operating at an output in the range of values between points 28d and 28a immediately following an operation at an output higher than represented by 28b and also prevents its operating at an output in the range of values between points 282 and 280 immediately following operation at a lower level. This situation is roughly comparable to that of an automobile that is unable to operate in the range between 5 and 25 miles per hour. This would be intolerable and the comparable inability of the polyphase bridge connected amplistat to operate stably and continuously at an output voltage or current within a predetermined range'of light load values is equally intolerable.

I have discovered that by modifying the exciting characteristic of the amplistat to impart to it an amount of skew within a predetermined range of values, both the snap action instability and the wave train instability are eliminated from the operation of amplistats of the character herein'described, i.e., polyphase bridge connected amplistats with rectangular B-H loop core material.

In accordance with the invention, stable operation of the amplistat is achieved by means of an appropriate combination of inductors with the amplistat which produces a composite exciting characteristic having the requisite amount of skew with respect to the unmodified rectangular characteristic. As shown in FIG. 1, a plurality of inductors 34, 35, 36, 37, 38 and 39 are employed. The inductor 34 comprises a winding 34a mounted on a ferromagnetic core member 34b and its winding is connected in a circuit in parallel with the magnetic amplifier gate winding 7. The core member 34b may, if desired, have an air gap for the purpose of improving the linearity of its characteristic and for the purpose of fixing the inductance of the device. The inductors 35-39 inclusive are similar to inductor 34 and their inductive windings are similarly connected in circuits in parallel with the gate windings of corresponding sectional magnetic amplifiers of the amplistat.

The exciting characteristic of the inductors may be either linear or non-linear. In FIG. 7 a suitable exciting characteristic is graphically illustrated by the curve 40. It is non-linear, although for a considerable range of intermediate values its non-linearity is relatively small. The manner in which a skew of the composite characteristic is produced by the combination of inductors with the magnetic amplifiers is graphically illustrated in FIG. 7 in which the curve 27 represents the unmodified rectangular exciting characteristic of the amplistat, curve 40 represents the exciting characteristic of the inductors 34 to 49 inclusive, and 41 represents the composite exciting characteristic. Since the inductor coils are connected in parallel with the gate windings of the magnetic amplifiers the volt-seconds applied to the gate winding and to the iductor windings are equal at every instant of a flux excursion. Viewing the amplistat and the inductors from the control winding of the amplistat, as a voltage is applied to a gate winding and thus to its parallel inductor, the control winding current necessary to provide the excitation is equal to the current necessary for providing the magnetomotive force for the amplistat plus the transformed value of the current necessary to provide the magnetomotive force for the inductor. Thus, the exciting characteristic of the composite device, viewed from the control winding of the amplistat is the sum of the two exciting currents at each value of the volt seconds applied to the gate winding and the inductor winding. Consequently, the composite characteristic 41 is obtained by adding the abscissae of curve 40 to the abscissae of curve 27 for each value of volt seconds applied to the windings. In order to produce a skew equal to the amount present and indicated in the composite exciting characteristic 41 in FIG. 7, and assuming the inductor to have a linear characteristic, the inductor is constructed to have an inductance defined by the following equation:

in which:

Nznumber of turns of gate winding A(fe):efiective area of cross section of core of associated magnetic amplifier Brzresidual flux density in such associated core membet at H Hczhalf width in oersteds of the dynamic loop of the associated core member and L(fe) :mean length in centimeters of term-magnetic path of the associated core member.

In the case of a non-linear characteristic the inductor is constructed to have an effective inductance (L equal to the value of L as determined by Equation 1 for the case of the linear inductor. Effective inductance is defined as an inductance that requires the current to change from zero to the same precise value of peak current that would result if a linear inductance of the same numerical value as the effective inductance were employed, and in each case the volt seconds acting to produce the current change is equal to the volt seconds necessary to raise the flux density from Zero to residual flux density when acting on the core of the amplistat by means of the gatewinding.

The precise value of L defined by Equation 1 is not at all critical. The skew of the composite exciting characteristic 41 may be increased to twice the amount illustrated in FIG. 7 without seriously reducing the gain of the amplistat. Such an increase in skew is obtained if the inductor is constructed to have one-half the inductance prescribed by Equation 1. Conversely the skew may be decreased by increasing the inductance. Increasing the inductance to ten times the value defined by Equation 1 reduces the skew to one-tenth the amount illustrated in FIG. 7. This amount of skew is effective to produce a substantial decrease in the snap action and wave train instability of an amplistat of the character herein described.

Another manifestation of instability results from the effect of inductive loads on the operation of the amplistat. It occurs in single phase and polyphase amplistat systems. The degree of severity of inductive load instability is closely related to the ratio of the load time constant to the period of the supply frequency.

In FIG. 8 is illustrated an amplistat control system in which a single phase amplistat 42 is connected in a diametric configuration to supply a highly inductive load 43. The amplistat gate windings 44 and 45 are connected to opposite terminals of the secondary winding 46a of a single phase supply transformer 46. Uncontrolled rectifiers such as diodes 47 and 48 are connected between the gate windings and terminal 43a of the inductive load 43 of which the other terminal 43b is connected to the center tap of the supply transformer secondary winding. The gate winding 44 is mounted on one leg 49 of a core member and similarly, gate winding 45 is mounted on a leg 50 of a core member. Also mounted on the core is a control winding having a coil 51 mounted on core leg 49 and a coil 52 mounted on core leg 50. These control winding coils are illustrated as being connected in series relationship to a source of control signals.

In order to counteract the destabilizing efiect of the highly inductive load, a series combination of a commutating rectifier 53 and a feedback winding comprisingseries connected coils 54 and 55 is connected in a circuit across the output or load terminals of the amplistat. The feedback coil 54 is mounted on core leg 49 and feedback coil 55 is mounted on core leg 50. These coils carry the current of the commutating rectifier and are connected in such polarity that they produce degeneration of the amplistat as shown by the polarity dots associated with the gate winding coils 44 and 45 and with the feedback winding coils. The commutating rectifier is connected so that it blocks in the direction of current flow from the positive output terminal of the amplistat to the negative output terminal; in other words the cathode of the commutating rectifier is connected to the positive output terminal and the anode of the commutating rectifier is connected to the negative output terminal.

For complete elimination of inductive load instability the total number of turns of the feedback winding coils should be in the range between 5% and 10% of the number of turns of the gate windings. By means of the commutating rectifier and degenerative feedback combination, elimination of snap-action instability due to inductive load is accomplished with only a slight reduction in gain and a correspondingly slight reduction of the quantity known as the amplistats figure of merit which is defined as a fraction of which the numerator is the power gain of the amplistat and the denominator is its time constant. This combination isthe subject matter of my co-pending divisional patent application S.N. 442,698, filed March 25, 1965, and assigned to the assignee of the present application.

Although a particular embodiment of the invention has been shown and described various modifications and changes will readily occur to those skilled in the art with out departing from the true spirit of the invention or from the scope of the annexed claims.

What I claim as new and desire to secure by Letters Patent of the United States is:

1. In combination,

(a) a plurality of self saturating magnetic amplifiers connected in polyphase bridge configuration with one ofsaid amplifiers in each arm of the bridge,

(h) each of said amplifiers having a rectangular exciting characteristic and comprising a core member having a rectangular hysteresis loop characteristic and having mounted thereon in inductive relationship a gate winding and a control winding,

(c) a plurality of inductors each comprising a magnetic core member having a sloping generally linear BH characteristic and a winding mounted thereon connected in a circuit in parallel with a corresponding one of said gate windings,

(d) and each of said inductors having a sloping generally linear volt seconds v. current exciting characteristic in which the exciting current is smoothly modulated with respect to volt seconds throughout the volt seconds range of said rectangular exciting characteristic thereby to produce a composite exciting characteristic that is skewed with respect to said rectangular exciting characteristic.

2. In combination,

(a) a plurality of self saturating magnetic amplifiers connected in polyphase bridge configuration with one of said amplifiers in each arm of the bridge,

(b) each of said amplifiers having a rectangular exciting characteristic and each comprising a core member having a rectangular hysteresis loop characteristic and having mounted thereon in inductive relationship a gate winding a control Winding,

(c) a plurality of inductors each having a substantial average inductance throughout a complete excursion between positive and negative saturation of the magnetic flux in said amplifier core,

(d) each of said inductors comprising a magnetic core having a sloping generally linear BH characteristic and having a winding mounted thereon and connected in parallel with a corresponding one of said gate windmgs,

(e) and each of said inductors having a sloping generally linear volt seconds v. current exciting characteristic in which the exciting current is continously modulated with respect to volt seconds throughout the volt seconds range of said rectangular exciting characteristic thereby to produce a composite exciting characteristic having a substantial skew with respect to said rectangular exciting characteristic.

3. Stabilized magnetic amplifying apparatus comprising,

(a) a plurality of self saturating magnetic amplifiers connected in polyphase bridge configuration with one of said amplifiers in each arm of the bridge,

(b) each of said amplifiers having a rectangular exciting characteristic and each comprising a magnetic core member having a rectangular BH loop characteristic and having a gate winding and a control winding mounted in inductive relationship thereon,

(c) a plurality of linear inductors each comprising a magnetic core member having a sloping generally linear BH characteristic and having a winding mounted thereon and connected in parallel with a corresponding one of said gate windings,

(d) and each of said inductors having a sloping generally linear volt seconds v. current exciting characteristic in which the exciting current varies substantially linearly with respect to volt seconds continuously throughout the volt seconds range of said rectangular exciting characteristic thereby to roduce a composite exciting characteristic having a substantial skew with respect to said rectangular exciting characteristic.

4. Stabilized magnetic amplifying apparatus comprising,

(a) a plurality of self saturating magnetic amplifiers connected in polyphase bridge configuration with one of said amplifiers in each arm of the bridge,

(b) each of said amplifiers having a rectangular exciting characteristic and each comprising a magnetic core member having a rectangular BH loop characteristic and having a gatewinding and a control winding mounted thereon in inductive relationship,

(c) a plurality of'choke coils each comprising a magnetic core member having a generally linear BH characteristic and having a Winding mounted thereon and connected in parallel with'a corresponding one of said gate windings,

(d) each of said choke coils having an average inductance in henries within the range between .2 and 4 times the quantity defined by W in which A(fe) is the eifective area of the cross section of the core of'the associated magnetic amplifier, N is the number of turns of the associated gate winding, Br is the residual flux density in said associated core member and L(fe) is the length in centimeters of the ferromagnetic path of said associated core member,

(e) and each of said choke coils having a sloping volt seconds v. current exciting characteristic in which the exciting current varies substantially linear with respect to volt seconds throughout the volt seconds range of said rectangular exciting characteristic of said associated magnetic amplifier thereby to produce a composite exciting characteristic having a substantial skew with respect to said rectangular exciting characteristic.

5. Stabilized magnetic amplifying apparatus comprising,

(a) a plurality of self saturating magnetic amplifiers connected in polyphase bridge configuration,

(b) each of said amplifiers having a rectangular exciting characteristic and each of said amplifiers comprising a magnetic core member having a rectangular BH loop characteristic and having a gate winding and a control winding mounted in inductive relationship thereon,

(c) a plurality of inductors each comprising a magnetic core member having a sloping generally linear BH characteristic and having a winding mounted thereon and connected in parallel with a correspond- 7 ing one of said gate windings,

(d) each of said inductors having throughout an excursion between positive and negative saturation of the magnetic flux of said rectangular BH characteristic cores an average substantially constant inductance in henries approximately equal to the quantity defined by wherein A(fe) is the effective area of the cross section of the core of the associated mganetic amplifier, N is the number of turns of the associated gate winding, Br is the residual flux density in said associated gate core member at H=0, H0 is the half width in oersteds of the dynamic loop of said associated core member and L(fe) is the mean length in centimeters of the ferromagnetic path of said associated core 'member,

(e) and each of said inductors having a sloping voltseconds v. current exciting characteristic in which the exciting current varies substantially linearly with respect to volt seconds throughout the volt seconds range of said rectangular exciting characteristic of its corresponding magnetic amplifier thereby to produce a composite exciting characteristic having a substantial skew with respect to said rectangular characteristic.

References Cited by the Examiner UNITED STATES PATENTS 2,777,021 1/57 Walker 323--89.9

LLOYD McCOLLUM, Primary Examiner.

ORIS L. RADER, Examiner. 

1. IN COMBINATION, (A) A PLURALITY OF SELF SATURATING MAGNETIC AMPLIFIERS CONNECTED IN POLYPHASE BRIDGE CONFIGURATION WITH ONE OF SAID AMPLIFIERS IN EACH ARM OF THE BRIDGE, (B) EACH OF SAID AMPLIFIERS HAVING A RECTANGULAR EXCITING CHARACTERISTIC AND COMPRISING A CORE MEMBER HAVING A RECTANGULAR HYSTERESIS LOOP CHARACTERISTIC AND HAVING MOUNTED THEREON IN INDUCTIVE RELATIONSHIP A GATE WINDING AND A CONTROL WINDING, (C) A PLURALITY OF INDUCTORS EACH COMPRISING A MAGNETIC CORE MEMBER HAVING A SLOPING GENERALLY LINEAR B-H CHARACTERISTIC AND A WINDING MOUNTED THEREON CONNECTED IN A CIRCUIT IN PARALLEL WITH A CORRESPONDING ONE OF SAID GATE WINDINGS, (D) AND EACH OF SAID INDUCTORS HAVING A SLOPING GENERALLY LINEAR VOLT SECONDS V. CURRENT EXCITING CHARACTERISTIC IN WHICH THE EXCITING CURRENT IS SMOOTHLY MODULATED WITH RESPECT TO VOLT SECONDS THROUGHOUT THE VOLT SECONDS RANGE OF SAID RECTANGULAR EXCITING CHARACTERISTIC THEREBY TO PRODUCE A COMPOSITE EXCITING CHARACTERISTIC THAT IS SKEWED WITH RESPECT TO SAID RECTANGULAR EXCITING CHARACTERISTIC. 